2 Detecting infection

As we noted earlier, one of the best ways of measuring the rate of infection in a community is by regularly testing a panel of individuals who are representative of that community. In the earliest stages of the COVID-19 pandemic, it was clear that some individuals could be infected but have no symptoms, but it was not certain whether this was a large or small proportion of the population. This information was, however, very important, since asymptomatic people were more likely to go about their day-to-day activities as normal in the community and potentially infect those they interacted with. This information also informed public health measures such as lock-downs, isolation periods and the wearing of masks.

The incidence of a disease such as COVID−19 is measured by detection of the pathogen, for example by PCR, lateral flow tests or laboratory culture of the infectious agent. These tests show positive for a limited period following infection – typically ~2 weeks for COVID-19. However, if infected people have no symptoms, they are unlikely to go forward for testing. Consequently, measuring the number of infections in people presenting to their doctor or a testing centre is likely to underestimate the true incidence of the disease in the community.

An alternative to detection of the pathogen is to measure antibodies against the pathogen.